ID 325064 - Mari crashes when flattening channel in specific archive

Problem summary
Crash when flattening channel in specific archive

Customer reported version
3.3v1

Customer reported platform
windows10

Steps to reproduce
1) Open the attached Archive
2) In the Channel Palette, choose 'channel 6 copy'
3) Right Click > Flatten
Result: Mari crashes

Workaround
n/a

Reproduced by support
Reproduced (in Safer mode). Mari logs (Windows and Mac) along with .dmp attached below:
3.3v1 - Win 10 - OSX 10.12.3 - CentOS 7.4
3.2v1 - Win 10 - OSX 10.12.3 - CentOS 7.4
3.1v3 - Win 10 - OSX 10.12.3 - CentOS 7.4
3.1v1 - Win 10 - OSX 10.12.3 - CentOS 7.4
3.0v4 - Win 10 - OSX 10.12.3 - CentOS 7.4
3.0v1 - Win 10 - OSX 10.12.3 - CentOS 7.4

Unable to reproduce (Safer included):
2.6v5 - Win 10 - OSX 10.12.3 - CentOS 7.4

Earliest version tested
2.6v5 - crash no longer occurs

Expected behaviour
Flatten channel when selecting 'Flatten'

Actual behaviour
Mari crashes